1. Field of the Invention
This invention relates generally to the field of charged particle trapping and more specifically to the use of a charged-particle trap to repetitively measure charged particles for mass spectrometry.
2. Description of Related Art
Electrospray ion sources are capable of generating high molecular weight (&gt;1 MDa) multiply-charged ions. Measuring the mass of megadalton ions is possible using one of two mass spectrometry techniques. The first relies on Fourier Transform Ion Cyclotron Resonance ("FTICR") and the second utilizes the simultaneous measurement of charge and time of flight.
In the FTICR method ions are ejected into a trapping cell where the resonance condition defined by the magnetic and radio frequency fields definitively resolve the mass to charge ratio ("m/z") of the trapped ions. It is possible to determine the mass of the trapped ions by analyzing their various m/z states. The high resolution achieved with FTICR suggests that the numerous m/z states for electrospray ions exceeding 1 MDa should be resolved (J. E. Bruce et al., Trapping, Detection, and Mass Measurement of Individual Ions in a Fourier Transform Ion Cyclotron Mass Spectrometer, J. Am. Chem. Soc., 116:7839, 1994). In practice, this goal had been confounded by heterogeneity of the population of trapped ions. An FTICR technique has been developed for analyzing individual electrospray ions thus avoiding the problem of heterogeneity. (X. Cheng et al., Charge-State Shifting of Individual Multiply-Charged Ions of Bovine Albumin Dimer and Molecular Weight Determination Using An Individual-Ion Approach, Anal. Chem. 66:2084, 1994). Currently however FTICR techniques are not well suited for rapidly analyzing a large number of individual ions sequentially, as is required for determining the average mass of a population of megadalton ions in a sample. FTICR techniques are also very expensive, requiring the use of large, complex instrumentation, including heavy magnets and ultra-high vacuum technology capable of achieving operating pressures of 10.sup.-11 to 10.sup.-12 Torr.
The second technique for megadalton ion mass spectrometry is described by Fuerstenau et al. in copending patent application Ser. No. 08/749,837, now U.S. Pat. No. 5,770,857. A low noise charge sensitive amplifier is used to capture the image charge of an ion accelerated through a known voltage V as it passes through a metal detector tube. The image charge signal comprises a pulse which rises when the ion enters the tube and falls when the ion exists the tube. The ion time of flight is measured from the pulse rise and fall, from which the ion's velocity is calculated. The mass to charge ratio of the ion, m/z, is calculated from the particle's time of flight when accelerated through a known electrostatic field. Simultaneously, the charge z of the ion is determined from the amplitude of the differentiated image charge signal, which is proportional to the ion's charge. With z known, the mass is calculated by multiplying the m/z and z.
The inventive mass spectrometer disclosed in copending patent application Ser. No. 08/749,837 measured ions making a single pass through a tube detector. Several thousand ions were analyzed in a few minutes, thus supplying enough data for calculating statistically significant measurements of the mass of molecules in a sample population. The cost advantage of this technology, when compared to FTICR, was obvious because large magnets and ultra-high vacuum were not needed. These two advantages were balanced, however, by the low precision of the single-pass charge detection approach. Depending on amplifier noise and the magnitude of the image charge, error in both the amplitude and timing measurements lead to fairly accurate but imprecise mass values.
In the one-pass format, the dominant cause of low mass resolution observed for megadalton DNA is due to imprecision of the charge measurement. An estimate of the relative errors associated with charge and velocity measurements can be determined using an electronic pulser to generate charge signals that simulate DNA ions flying through the detector tube. The use of a pulser eliminates measurement variations caused by fluctuations of ion charge and velocity. By introducing 10 .mu.s wide 0.5 mV pulses into the charge-sensitive preamplifier, as typically produced by transiting 3 MDa ions formed by positive mode electrospray, the relative standard deviation (n=100) of the charge measurement is 0.054 compared to a relative standard deviation of 0.013 for the velocity measurement. These values illustrate the relative importance of the charge determination in limiting the precision of the overall measurement.
Time of flight ("TOF") mass spectrometers are instruments that measure the mass of ions by measuring the time they take to traverse a fixed distance. Typically these spectrometers have a source region where ions are formed and accelerated through a potential, a field free drift region, and a detector at the end of the drift region. A problem arises if the ions do not all have the same energy. Higher energy ions arrive at the detector ahead of lower energy ions having the same mass. This spreading of flight times limits the mass resolution of the spectrometer.
B. A. Mamyrin et al. described a time focusing ion mirror, which they term a "reflectron". Their ion mirror defocuses the ion beam in order to preserve the time resolution necessary for time-of-flight (TOF) spectroscopy (Soviet Physics JETP, 37(1973)4S). The Mamyrin reflectron comprises of a series of metal rings to which separate voltages are applied to establish an electrostatic field capable of reflecting incident ions about an axis of symmetry and in a plane normal to the plane of the mirror. The voltages applied to the metal rings that make up the reflectron create a flat electrostatic field between the rings. The reflectron causes ions having different energies but the same mass to arrive at the detector at the same time. Reflectrons are not used for spatial focusing, rather they depend on spatial defocusing to preserve the time resolution.